1. Field of the Invention
This invention relates broadly to the field of renewable energy, and more specifically to systems and methods for fabricating photovoltaic cells and components therefor.
2. Description of the Related Technology
Silicon-based photovoltaic cells have been used for converting light into electricity now for close to 70 years. U.S. Pat. No. 2,402,662 issued on Jun. 25, 1946 (application date May 27, 1941) describes a device made from two sections of the same silicon ingot, one with a columnar structure and the other with a non-columnar structure, those sections having been fused together to form a “Light-Sensitive Electrical Device”. In 1954 Bell Laboratories further discovered that sections from silicon ingots doped with different impurities such as boron and phosphorous and fused to each other resulted in a device even more sensitive to light.
The most common material used for manufacturing photovoltaic (PV) cells or solar cells as they are commonly called in the solar industry is the metalloid “silicon,” which is the 8th most common element in the universe by mass and the 2nd most abundant element found in the Earth's crust (after oxygen), forming 25.7% of the Earth's crust by mass.
The most common method of manufacturing silicon-based photovoltaic solar cells is by slicing crystalline silicon billets or boules into thin wafers. One of the boules contains traces of boron or another Group III acceptor as the impurity, making it the p-type (positive) material and the other contains traces of phosphorus or another Group V type donor as the impurity making it the n-type (negative) material. Once wafers from each of these boules are fused together and further processed, they form the semi-conductor device known as a photovoltaic cell.
The conventional process of manufacturing a PV cell is very complicated, with multiple steps such as sawing the silicon billets into wafers, grinding the surfaces, etching the surfaces, fusing them together, and so forth. The physical size of the silicon wafer is limited by the billet size and has evolved over time from 25.4 mm (1 inch) to 300 mm (11.8 inches). Efforts are now underway to increase the size from 300 mm to 450 mm (17.7 inches).
A more recent manufacturing process for silicon-based PV solar cells was first described in U.S. Pat. No. 4,661,200, issued on Apr. 28, 1987 as a “String Stabilized Ribbon Growth” and later in U.S. Pat. No. 4,689,109 issued Aug. 25, 1987 as a method of initiating or seeding the growth of a crystalline or polycrystalline ribbon. This process draws a pair of strings or filaments through a shallow crucible of molten silicon, which spans the space between them like a wand through a soap solution forming a “ribbon” of thin silicon. This eliminates the step of sawing the silicon wafers from a solid billet or boule of silicon, which in turn can reduce the 45% loss of expensive silicon material that occurs during the sawing process.
The string ribbon process permits manufacture of an effective solar cell with enormous savings of time and material. That represents a major step in reducing the cost of solar energy to make it more cost competitive with the current carbon-based methods of mass producing electricity. However, the scale of solidifying a ribbon between two strings has obvious limitations that prevent it from being the quantum leap needed to move the solar industry from its current position as an alternative energy source to one of being a primary energy source in the world today.
Twin-roll casting, or drum casting as it is sometimes called, is a well-known continuous casting process for manufacturing steel and aluminum sheets. The original concept is described in U.S. Pat. No. 49,053 dated Jul. 25, 1865 as “An improvement in the manufacture of iron and steel.” However, the process did not reach its full potential until the 21st century when computerized control systems were able to accurately match the feed rate of the liquid metal pool with the speed of the rotating drums. Continuous casting processes such as twin-roll casting permit the efficient manufacture of large quantities of metallic materials having a relatively thin form factor.
U.S. Pat. No. 4,108,714 to Keller et al. discloses the fabrication of a wide ribbon of silicon sheet material, to be used in the manufacture of solar cells, from a molten silicon stream passing through the nip between spaced apart rollers. However, manufacture of solar cells using this process would require two sheets of such material be subjected to a number of additional manufacturing steps, including doping the sheets, grinding the surfaces, etching the surfaces and fusing them together in order to create the necessary p-n junction.
A need exists for a system and method that will permit the large scale fabrication of photovoltaic units having a p-n junction more cost-effectively than has heretofore been achieved.